1. Field of the Invention
The invention relates to a liquid crystal display device.
2. Description of the Related Art
Japanese Patent Application Publication No. 1-270024A has suggested a liquid crystal display device comprised of a liquid crystal cell in which liquid crystal is vertically aligned, first and second polarizer plates arranged at opposite sides about the liquid crystal cell, a first λ/4 plate arranged between the liquid crystal cell and the first polarizer plate, and a second λ/4 plate arranged between the liquid crystal cell and the second polarizer plate, wherein “λ” indicates a wavelength of a light. The suggested liquid crystal display device improves viewing angle characteristic observed when a viewer obliquely views the liquid crystal display device.
There has been developed a display presenting high visibility even under external light. For instance, Japanese Patent Application Publication No. 2000-29010A (Japanese Patent No. 3410663) has suggested a liquid crystal display device having both a first area in which a light passes therethrough for displaying images and a second area in which a light is reflected for displaying images, and improving characteristic for dark-displaying. The suggested liquid crystal display device is structurally different from the liquid crystal display device suggested in Japanese Patent Application Publication No. 1-270024A in that the liquid crystal cell has the first and second areas. The liquid crystal cell having liquid crystal vertically aligned is used in a normally-black mode in which darkness is displayed when a voltage is not applied thereto and illumination is displayed when a voltage is applied thereto. The suggested liquid crystal display device improves contrast by including normally-black vertically-aligned liquid crystal, in comparison with including TN (twisted nematic) mode liquid crystal or STN (super twisted nematic) mode liquid crystal.
Japanese Patent Publication No. 7-69536B has suggested a liquid crystal display device comprised of a liquid crystal cell in which liquid crystal is vertically aligned, first and second polarizer plates arranged at opposite sides about the liquid crystal cell, a first λ/4 plate arranged between the liquid crystal cell and the first polarizer plate, a second λ/4 plate arranged between the liquid crystal cell and the second polarizer plate, a first negative uniaxial optical compensation layer arranged between the first λ/4 plate and the liquid crystal cell, and a second negative uniaxial optical compensation layer arranged between the second λ/4 plate and the liquid crystal cell. The suggested liquid crystal display device improves viewing angle characteristic for dark-displaying.
Japanese Patent Application Publication No. 2002-303869A has suggested a liquid crystal display device having the same structure as that of the liquid crystal display device suggested in Japanese Patent Publication No. 7-69536B, but being designed as a multi-domain type liquid crystal display device. Herein, a multi-domain type liquid crystal display device indicates a liquid crystal display device in which each of pixels has a plurality of domains in which alignment conditions of liquid crystal are different from one another.
The above-mentioned liquid crystal display device suggested in Japanese Patent Application Publication No. 1-270024A is accompanied with a problem of poor symmetry in contrast, and the liquid crystal display device suggested in Japanese Patent Application Publication No. 2000-29010A improves symmetry in contrast, but is accompanied with a problem that light is leaked when a viewer varies a viewing angle from a direction in which a normal line of a substrate extends.
The liquid crystal display devices suggested in Japanese Patent Publication No. 7-69536B and Japanese Patent Application Publication No. 2002-303869A are accompanied with a problem that desired viewing angle characteristic cannot be obtained.
Japanese Patent Application Publication No. 2002-303869A sets forth that a direction in which contrast is best maintained is obtained by rotating 0 (X-axis direction), 90, 180 and 270 degrees directions by about 30 degrees in a counterclockwise direction, and that contrast is maximized at 0 (X-axis direction), 90, 180 and 270 degrees directions by optimizing angles of the polarizers. However, the Publication fails to indicate a reason why a direction in which contrast is best maintained is rotated, and a relation among retardation of a liquid crystal layer, retardation of a compensation layer, and an arrangement angle of polarizers.
Japanese Patent Application Publication No. 2001-343653 has suggested a liquid crystal display device including a first substrate on which a pixel electrode is formed, a second substrate on which an opposing electrode is formed, a liquid crystal layer sandwiched between the first and second substrates, and containing liquid crystal molecules aligned in different directions in each of pixels when a voltage applied across the pixel and opposing electrodes is equal to a predetermined voltage, a photodetector detecting a light having passed through the liquid crystal layer, a polarizer for polarizing a light entering the liquid crystal layer into a circularly polarized light, and a first retardation layer arranged between the liquid crystal layer and the photodetector, and having in-plane retardation equal to a quarter of a wavelength of a light passing through the liquid crystal layer.
Japanese Patent Application Publication No. 2002-55342 has suggested a liquid crystal display device including a liquid crystal cell, polarizers arranged at opposite sides of the liquid crystal cell, λ/4 layers arranged between the polarizers and the liquid crystal cell, and having retardation almost equal to a quarter of a wavelength of a light passing through the liquid crystal cell, a retardation layer arranged between the liquid crystal cell and one of the λ/4 layers, having retardation in a thickness-wise direction thereof, and optically compensating for the liquid crystal cell, and a compensation layer arranged between the polarizer and the above-mentioned one of the λ/4 layers, wherein the retardation of the compensation layer in a thickness-wise direction thereof has a sign opposite to that of a sum of the retardation of the polarizer, the λ/4 layer and the retardation layer.
Japanese Patent Application Publication No. 2003-114428 has suggested a liquid crystal display device including a first substrate having an area in which an incident light is reflected and an area in which a light passes therethrough, a second substrate on which an opposing substrate is formed, and a liquid crystal layer sandwiched between the first and second substrates, a first polarizer arranged on a surface of the second substrate oppositely to the liquid crystal layer, a second polarizer arranged on a surface of the first substrate oppositely to the liquid crystal layer, a first retardation plate arranged between the first polarizer and the liquid crystal layer, and a second retardation plate arranged between the second polarizer and the liquid crystal layer. Each of the first and second retardation plates is comprised of a λ/4 plate, a light-transmission axis of the first polarizer and a lagging axis of the first retardation plate form 45 degrees therebetween, and a light-transmission axis of the second polarizer and a lagging axis of the second retardation plate form 45 degrees therebetween.
Japanese Patent Application Publication No. 2003-207782 has suggested a liquid crystal display device is comprised of a liquid crystal cell including a pair of substrates, and liquid crystal vertically aligned to the substrates when no voltage is applied thereto, and a polarizer arranged outside the liquid crystal cell, and including a linearly polarizing plate, and an optical device composed of two-axes optically anisotropic material and having retardation equal to λ/4.